There are two major pumping chambers in the heart, the left and right ventricles. Contracting simultaneously, these chambers pump blood into the aorta and the pulmonary artery. Blood enters the ventricles from the left and right atria and is forced into the ventricles by an atrial contraction, which precedes a major ventricular contraction by an interval of about 100 milliseconds. This interval is known as the atrial ventricular (AV) delay. In a healthy heart, the atrial and ventricular contractions begin with a wave of electrical excitation that originates in the right atrium and spreads to the left atrium. This excitation of the cardiac muscle then spreads to the AV node, which delays its passage to the ventricles. Generally, such atrial excitations begin every 400-1,000 milliseconds at a metabolically-determined frequency known as the sinus rate. Sometimes however, in a diseased or damaged heart, such electrical excitation signals are either not properly produced or do not reach the heart muscle. In either case, the heart will not pump blood properly.
Traditionally, the cure for such a heart condition, when permanent, has been to implant an internal cardiac pacemaker that electrically supplies the necessary excitation pacing signals through a set of electrodes connected directly to the heart. Such electrodes may include a single electrode or pairs of electrodes that deliver the pacing currents to various areas of the heart muscle. Until a permanent cardiac pacemaker can be surgically implanted, a patient suffering from such a heart condition may need to have such pacing signals applied externally using a cardiac pacemaker in order to stay alive. Such pacing signals may be applied continuously if the heart is completely malfunctioning, or intermittently if it is only occasionally malfunctioning.
External cardiac pacers are also used to restore a temporarily malfunctioning heart to normal operation, as can occur, for example, after electrical shock. Generally, such an external pacer includes a set of electrodes that are placed in electrical contact with the patient's chest. The pacer delivers an electric current large enough to stimulate the heart muscle and contract it, thereby pumping blood.
The problem with such external cardiac pacers is that the pacing currents delivered to the external electrodes are sometimes sufficiently great to cause burning or severe discomfort and pain to the patient at the site where the electrodes are placed. To overcome this problem, attempts have been made to reduce the current density at the site where the individual pacing electrodes are placed during cardiac pacing. One such attempt involved the use of pacing electrodes having a large surface area. However, such electrodes did not prove successful in reducing patient discomfort. Therefore, it remains desirable to have a pacing system that will provide a pacing current to the heart muscle with a reduced current density to avoid causing severe burns or discomfort to the patient.